OZONE AM) HYDROGEN PEROXIDE DAI/TON'S LAW 201 



black (from oxidation) in. ozonised oxygen. It is rapidly absorbed by 

 mercury, forming oxide ; it transforms the lower oxides into higher for 

 instance, sulphurous anhydride into sulphuric, nitrous oxide into 

 nitric, arsenious anhydride (As. 2 O 3 ) into arsenic anhydride (As. 2 5 ) <fcc. 7 

 But what is especially characteristic in ozone is the decomposing action 

 it exerts on potassium iodide. Oxygen does not act on it, but ozone 

 passed into a solution of potassium iodide liberates iodine, whilst the 

 potassium is obtained as caustic potash, which remains in solution, 

 2KI + H 2 O + 0=2KHO-|-l2. As the presence of minute traces of 

 free iodine may be discovered by means of starch paste, with which it 

 forms a very dark blue coloured substance, a mixture of potassium 

 iodide with starch paste will detect the presence of very small traces of 

 ozone. 8 Ozone is destroyed or converted into ordinary oxygen not 

 only by heat, but also by long keeping, especially in the presence of 

 alkalis, peroxide of manganese, chlorine, tkc. 



Hence ozone, although it has the same composition as oxygen, differs 



7 Ozone takes up the hydrogen from hydrochloric acid ; the chlorine is set free, and 

 can dissolve gold. Chromium and iodine are directly oxidised by ozone, but not by oxygen, 

 and so also with a number of other substances. Ammonia, NH 5 , is oxidised by ozone into 

 ammonium nitrite (and nitrate), 2NH 5 + O 3 = NH 4 NO^ + H 2 O, and therefore a drop of 

 ammonia, on falling into the gas, gives a thick cloud of the salts formed. Ozone converts 

 lead oxide into peroxide, and suboxide of thallium (which is colourless) into oxide (which 

 is brown), so that this reaction is made use of for discovering the presence of ozone. 

 Lead sulphide, PbS, is converted into sulphate, PbSO 4 , by ozone. A neutral solution of 

 manganese sulphate gives a precipitate of manganese peroxide, and an acid solution may 

 be oxidised into permanganic acid, HMnO 4 . With respect to the oxidising action of ozone 

 on organic substances, it may be mentioned that with ether, C 4 H 10 O, ozone gives ethyl 

 peroxide, which is capable of decomposing with explosion (according to Berthelot), and is 

 decomposed by water into alcohol, 2C.>H e O, and hydrogen peroxide, EUO.j. 



8 This reaction is the one usually made use of for detecting the presence of ozone. 

 In the majority of cases paper is soaked in solutions of potassium iodide and starch. 

 Such ozonometrical or iodised starch-paper when damp turns blue in the presence of ozone, 

 and the tint obtained varies considerably, according to the length of tune it is exposed and 

 to the amount of ozone present. The amount of ozone in a given gas may even to a 

 certain degree be judged by the shade of colour acquired by the paper, if preliminary 

 tests be made. 



Test-paper for ozone is prepared in the following manner: One gram of neutral 

 potassium iodide is dissolved in 100 grams of distilled water; 10 grams of starch are 

 then shaken up in the solution, and the mixture is boiled until the starch is converted 

 into a jelly. This jelly is then smeared over blotting-paper and left to dry. The colour 

 of iodised staivh-paper is changed not only by the action of ozone, but of many other 

 oxidisers ; for example, by the oxides of nitrogen and hydrogen peroxide. Houzeau pro- 

 posed soaking common litmus-paper with a solution of potassium iodide, which in the 

 presence of iodine would turn blue, owing to the formation of K.HO. In order to find if 

 the blue colour is not produced by an alkali (ammonia) in the gas, a portion of the papt-r 

 is not soaked in the potassium iodide, but moistened with water ; this portion will then 

 also turn blue if ammonia be present. A reagent for distinguishing ozone from hydrogen 

 peroxide with certainty is not known, and therefore these substances in very small quan- 

 tities (for instance, in the atmosphere) may easily be confounded. 



